The color quality of a light source relates to the ability of the light source to faithfully reproduce the colors of objects illuminated by the light source, in comparison with natural light. As expected, the color quality of the light source is an important characteristic of the light source in general, and to consumers in particular. Most consumers want an object that appears red in natural light to appear the same color of red when illuminated by the light source. For example, a light source with poor color quality may cause the red object to appear anywhere from orange to brown when illuminated.
The Color Rendering Index (CRI) is a measure of the relative color quality of a light source with respect to natural light. The CRI is the only internationally accepted standard for measuring color quality and is defined by the International Commission on Illumination (CIE or Commission internationale de l′eclairage). At a high level, the CRI for a light source is calculated by initially measuring the color appearance of 14 reflective samples of different defined hues under both a reference source and the light source being measured. The measured color appearances are then modified for chromatic adaptation with a Von Kires correction. After modification, the difference in the color appearance for each reflective sample i is referred to as the color appearance difference, ΔEi.
Based on the corresponding color appearance difference, ΔEi, a special CRI, Ri, is calculated for each reflective sample using the formula: Ri=100−4.6ΔEi. To calculate the general CRI, Ra, for the light source, an average of the special CRI, Ri for only the first eight of the reflective samples is calculated, wherein:
      R    a    =            1      8        ⁢                  ∑                  i          =          1                8            ⁢              R        i            A perfect CRI of 100 indicates that there are essentially no color differences for any of the eight reflective samples that are used to calculate the general CRI Ra.
For reference, natural sunlight has a high CRI Ra of approximately 100, and incandescent light has a CRI Ra of 95 or greater. Florescent lighting is less accurate and generally has a CRI Ra of 70-80, which is on the lower end of what is acceptable for residential and indoor commercial lighting applications. Street lamps that use mercury vapor or sodium lamps often have a relatively poor CRI Ra of around 40 or lower.
The CRI of a light source only considers color rendering, as the name implies, and ignores many other attributes that impact overall color quality, such as chromatic discrimination and common observer preferences. Even as a measure of color rendering, CRI is calculated using only eight of the 14 reflective samples, as noted above. These eight reflective samples are all of low to medium chromatic saturation and do not span the range of normal visible colors. Thus, the CRI calculations do not take into consideration the ability of the light source to properly render highly saturated colors. As a result, light sources that render colors of low saturation well, but perform poorly with highly saturated colors can achieve relatively high CRIs, while light sources that afford high chromatic discrimination are pleasing to the common observer, and perform relatively well for colors at all saturation levels may have a relatively low CRI.
The use of the CRI as a reliable color quality metric for solid-state lighting sources, such as those employing light emitting diodes (LEDs), is particularly problematic given the inherently peaked light spectrum of LEDs. Depending on how the spectrum of a given LED light source aligns with the reflective samples used to calculate the CRI, the resulting CRI may not be a fair representation of the perceived color quality of the LED light source in comparison with other LED light sources with different light spectra as well as with other traditional light sources. For example, a well-designed LED lighting source with a lower CRI Ra of 80 may be perceived as having a much more accurate and pleasing color rendering than a florescent lighting source with same CRI Ra of 80. Similarly, a first LED lighting source that is engineered to achieve a higher CRI Ra of 90 may not be perceived as being able to render colors as well as a second LED lighting source with a lower CRI Ra.
Given the limitations of the CRI as a measure of color quality for solid-state lighting devices, a new color quality metric, which is referred to as the Color Quality Scale (CQS), has been developed by the National Institute of Standards and Technology (NIST). Instead of using only eight low-chroma samples that do not span the full range of hues, the CQS takes in to consideration 15 Munsell samples that have much higher chroma and are spaced evenly along the entire hue circle. CQS also takes in to consideration various other characteristics that have been determined to impact an observer's perception of color quality. The CQS has a range of 0-100, with 100 being a perfect score. The details of how CQS is measured as of the date of filing is provided in Appendix A, an article entitled “Color Rendering of Light Sources,” from the National Institute of Standards and Technology web site (http://physics.nist.gov/Divisions/Div844/facilities/vision/color.html), accessed on Mar. 11, 2009 and incorporated herein by reference in its entirety.
Accordingly, CRI and CQI provide exemplary, but non-limiting, color rendering metrics upon which the color rendering of alight source are judged.